For example, in an image forming apparatus using an electrophotographic system, such as a facsimile, a printer or a copying machine, a toner is adhered to an electrostatic latent image formed on the surface of a photosensitive member to visualize it, the visualized image is transferred to a sheet or the like and thereafter it is fixed by being heated and pressurized. In terms of achieving high image quality and colorization, as a developer, a so-called two-component developer containing a carrier and a toner is widely used.
In a development system using a two-component developer, a carrier and a toner are agitated and mixed within a development device, and the toner is charged by friction so as to have a predetermined amount. Then, the developer is supplied to a rotating development roller, a magnetic brush is formed on the development roller and the toner is electrically moved to the photosensitive member through the magnetic brush to visualize the electrostatic latent image on the photosensitive member. The carrier after the movement of the toner is left on the development roller, and is mixed again with the toner within the development device. Hence, as the properties of the carrier, a magnetic property for forming the magnetic brush and a charging property for providing a desired charge to the toner are required. As such a carrier, a so-called coating carrier which is obtained by coating, with a resin, the surface of a carrier core material formed of magnetite, various types of ferrites or the like has so far been often used. The carrier core material which has so far been used for the coating carrier is formed in the shape of a perfect sphere.
In recent years, there has been a tendency that in order to cope with the market demand for increasing the speed of image formation in an image forming apparatus, the rotation speed of a development roller is increased such that the amount of developer supplied to a development region per unit time is increased.
However, in the coating carrier using the carrier core material in the shape of a perfect sphere, a failure is encountered in which the supply of the toner to the development region is insufficient and in which thus an image density is lowered. For example, a failure called development memory is encountered in which the image density is lowered by the influence of an image in the preceding revolution of the development roller.
Hence, a technology is proposed in which the surface of the carrier core material is formed in a concave-convex shape or different shapes of the carrier core material are formed, and in which thus frictional resistance to the surface of a photosensitive member and the frictional resistance of carriers are increased such that the amount of toner supplied to the development region is increased (for example, patent documents 1 and 2).